1. Technical Field
The present application relates to high build coatings, such as plaster and stucco, and more particularly to an apparatus for application of high build coatings.
2. Background of Related Art
Many industries, such as the construction and aerospace industries, have a need to apply high build coatings. High build coatings are coatings with a finished thickness from about 0.060" to 2.000" and weights ranging from approximately 2 lbs/cubic foot to 100 lbs/cubic foot. Plaster and stucco are examples of two high build coatings which are frequently used in the construction industry for aesthetic and structural purposes, as well as for fireproofing. Plaster and stucco coatings are mixtures of hygroscopic binders, fillers and water. Plaster is typically an interior coating based on gypsum, while stucco is traditionally an exterior coating which is usually based on portland cement, but also has been made with other hygroscopic materials such as gypsum. The fillers are typically materials such as cork, vermiculite, glass fibers, styrofoam beads, phenolic microballoons, glass microballoons, and cellulose fibers.
Traditional application techniques required manual mixing of the plaster and/or stucco in a barrel, or pot, with the mixed material being applied manually. Manual application had many disadvantages including a short pot-life of the material and the labor required to apply the material.
Because thickness up to about 2.00" are regularly required for high build coatings, spray systems have been developed to apply high build coatings more quickly than manual application allows. Prior art spray systems generally include a bin or hopper for holding a dry material which typically includes the hydroscopic material and a filler, a nozzle for wetting the dry material, a flexible conduit extending between the bin and the nozzle and an apparatus for generating a motive force to move the dry material from the bin or hopper through the conduit and out the nozzle. The prior art spray system may include two bins, one for holding the hydroscopic binder and one for the filler material, and a mixing device for combining the dry materials either prior to entry into the conduit or along the conduit, after entry.
Such prior art spray systems feed the dry material to the nozzle, which then wets the dry material, and ejects the wetted material onto the substrate thereby coating it. Typically the dry material comprises the hydroscopic material and a filler. Properties of the coating deposited are dependent on water control, the degree of wetting, uniformity of the hydroscopic material/filler distribution and filling timing.
One problem associated with typical prior art spray systems stems from the means used to generate the motive force. Two such means are a compressor and an eductor connected to the bin. The compressor forces the dry material out of the bin through the conduit and nozzle using pressurized air. The eductor creates a low pressure upstream of the eductor that draws the dry material out of the bin.
The means for generating the motive force can be placed at the upstream end of the conduit or the downstream end of the conduit. Eductors and compressors placed at the upstream end of the conduit push the dry material through the conduit. The problem with pushing the dry material through the conduit is that line losses are experienced, material separation according to particle size occurs, and fall-out occurs. All of which results in a non-uniform mixture passing through the nozzle which consequently inhibits wetting of the dry material and therefore decreases the properties of the coating.
To solve the problems of upstream eductors and compressors, eductors have been placed at the downstream end of the conduit, such as the granular material emitting means shown in U.S. Pat. No. 3,788,555. The granular material emitting means comprises an elongated, substantially tubular main body portion and an elongated tubular branch portion. The axis of the bore of the branch portion intersects the axis of the bore of the main body portion at an acute angle of about 30.degree. to about 80.degree., with 30.degree. to 60.degree. being preferable. One end of the bore of the branch portion is connected to a suitable source of fluid under pressure and one end of the tubular main body portion is connected to granular material reservoir through a conduit. During operation, the fluid under pressure flows through the branch portion and the acute angle is sufficient to cause the flow of the fluid to provide an area of reduced pressure upstream of the junction between the bore of the main body portion and the bore of the branch portion. This reduced pressure tends to cause the granular material to be withdrawn from the reservoir, entrained in the fluid, and carried through the conduit to the main body portion and out of the granular emitting means.
The downstream eductor of the aforementioned patent has the benefit of pulling the granular material all the way up the conduit, thus no line losses, material separation or fallout should be experienced. However, the downstream eductor above has two problems with respect to applying high build coatings. The granular material for use with the granular emitting device are perlite, clay, sand, talc, mica, calcium carbonate, calcium silicate, glass beads, plastic spheres and the like. These materials all weigh less than the hydroscopic materials necessary to form high build coatings; therefore, the motive force requirements necessary for the granular emitting means are less than those necessary for forming high build coatings.
Another problem associated with the spray apparatus disclosed in the aforementioned patent is that convergent nozzles are used to mix the granular material with a plural component material, such as a resin and a curing agent external to the spray apparatus. The use of convergent mixing external to the spray apparatus is necessary in the Harrison patent because the resin and curing agent turn from liquid to solid upon contact with the atmosphere; therefore mixing outside the spray apparatus is necessary to prevent clogging of the apparatus. However, it is desired that high build materials be mixed within the nozzle in order to optimize wetting and exercise greater control of pressure, turbulence and impingement angles.
A need therefore exists for a spray apparatus to apply coatings of hygroscopic material and filler, which exerts sufficient motive force on dry material to move it and which maintains this force throughout length of conduit, while substantially uniformly wetting the dry material within the nozzle.
The present application provides for a spray system including a conveyance device which moves the dry material through a conduit and uniformly wets the dry material within a nozzle.